CI: check formatting on the PR, not at release time

[scgopi]

The problem

Five releases in a row have reached main with swift-format --strict failing:

PR	Violations
#320	3
ProjectPersistence	3
#350	5
#351	1
#352	1

Every one was a line the formatter wraps on its own. Every one was found while cutting a release, and fixed there as a separate commit before the version bump.

The reason it keeps happening is not carelessness. Branch gates and Linux CI were green on all five — swift build passing and a full test run passing say nothing whatever about formatting, and nothing on the PR path runs make check. There is no signal until a release looks.

The change

One step in the Linux workflow, before swift build so the cheap check fails first:

- name: swift-format --strict
  run: swift format lint --recursive --strict --configuration .swift-format
    GraphcodeKit graphcode graphcode-cli graphcoded

That is the format half of make check, verbatim. swift format ships with the toolchain in the swift:6.2 container, so this adds no dependency and no install step.

The swiftlint half is deliberately left out: swiftlint is not in that image, and none of the five failures were lint — adding it would mean a new dependency for no demonstrated benefit.

Verified in both directions

A check that cannot fail is worse than no check, so I tested the real exit code rather than assuming:

• With a [LineLength] violation injected: exit 1, reporting the offending line
• Clean: exit 0

I also confirmed the YAML folds to exactly the intended single-line command, and reverted the injected violation so only the workflow file is changed here.

What this moves

Each of those five cost a release cycle: a red gate discovered mid-cut, a formatter run, a separate commit, a push, and a re-gate. This moves that to a failed check on the PR, where the author can fix it with swift format format --in-place in seconds.